1. Field of the Invention
The present invention is related to the field of digital electronic data telemetry systems. More specifically, the invention is related to methods of maintaining the synchronization of a transmitter and a receiver in a digital electronic telemetry system.
2. Discussion of the Relevant Art
Digital electronic telemetry systems are known in the art. Digital electronic telemetry systems are used to send information over a significant distance in the form of an electrical signal which comprises electrical voltage levels corresponding to binary numbers. Binary numbers are typically used in digital telemetry because they are composed entirely of ones and zeroes. Electrical voltage representations of ones and zeroes are relatively easy to receive and interpret, and are relatively resistant to corruption by noise, particularly when compared with analog telemetry signals.
Applications for digital electronic telemetry systems include transmission of measurements made by instruments such as oil well logging tools over a length of electrical cable. Oil well logging tools can include sensors which make measurements of physical parameters in a wellbore such as pressure and temperature. Typically, the sensors generate an output which can be in the form of an analog signal. The analog signal is converted into a series of binary numbers which represent the magnitude of the analog signal at spaced apart time intervals. The series of binary numbers is applied to a telemetry transmitter. The telemetry transmitter typically is programmed to send a plurality of such series of binary numbers, each series representing analog signal magnitudes of a corresponding sensor. The plurality of series are arranged so that each series typically is located at a predetemined ordinal position in a digital message called a telemetry sequence. Each time a telemetry sequence is transmitted, new series representing later sampled magnitudes of sensor signals can be transmitted in that sequence.
Each telemetry sequence typically starts with a predetermined series of binary numbers known as a synchronization pattern. The synchronization pattern serves two functions. First, detection of the synchronization pattern by a telemetry receiver indicates to the receiver that subsequent binary values detected from the telemetry sequence represent a new series of sensor magnitudes located in their respective predetermined positions. This enables the receiver to assign subsequently decoded values to the intended output destination for interpretation. Second, the synchronization pattern can be used to determine the exact transmitter frequency. This is particularly important in oil well logging tools since the telemetry transmitter, which forms part of a tool string, typically will be lowered into the wellbore and therefore can be exposed to elevated temperatures. Exposing the transmitter to different temperatures can cause changes in the operating frequency of the telemetry transmitter. Changes in the operating frequency can cause errors in decoding of the telemetry sequence by the receiver if the receiver frequency does not remain precisely matched to the transmitter frequency.
There are several methods known in the art for modulating the output of the transmitter to represent binary ones and zeroes. The form of modulation is referred to as the data encoding system. Data encoding systems typically used for telemetry in oil well logging tools, such as "alternate mark inversion" or AMI, can be susceptible to decoding error due to frequency mismatch between the transmitter and the receiver, because AMI and other encoding systems were principally designed for other telemetry uses in which the transmitter frequency was not subject to the range of variation to which the transmitter in oil well logging tools is susceptible. AMI telemetry, for example, codes a binary "one" as a non-zero signal level of substantially constant amplitude and at a much higher level than the ambient noise in the telemetry channel, and codes a "zero" as a zero signal level. Any binary "one" must be of a polarity opposite to the immediately previous "one" transmitted in the telemetry sequence. Alternating polarity of the "ones" maintains an average signal level in the telemetry channel, which in an oil well logging tool string is the cable, of close to zero.
Correct AMI telemetry decoding depends on the receiver sampling the telemetry signal at the proper position in time for each binary number. For example, a series of sixteen contiguous "zero" values in the telemetry sequence would be transmitted as a zero level signal for a period of time equal to sixteen times the period associated with the transmission of a single "zero". Since there are no time position reference markers within this particular telemetry sequence, other than the synchronization pattern at the beginning of the telemetry sequence, a slight mismatch in frequency between the transmitter and the receiver can cause misinterpretation of the received binary numbers. Misinterpretation of the received binary numbers can take forms such as the decoded sensor values being numerically incorrect, or decoded values being attributed to the wrong sensor.
A method of matching the frequency of the receiver to the frequency of the transmitter is known in the art. The method known in the art comprises transmitting a synchronization pattern consisting essentially of all binary "ones" to enable precise determination of the transmitter frequency by measuring the time between successive detections of the "ones". The frequency of the receiver is then adjusted by using an analog phase locked loop to match the transmitter frequency after detecting each synchronization pattern.
The method known in the art for matching the receiver frequency to the transmitter frequency is susceptible to loss of synchronization, and subsequent loss of data, even if small amounts of noise are present in the telemetry channel. For example, during synchronization, the receiver is programmed to search for voltage levels exceeding a detection threshold within a predetermined time window. The window has a primary duration functionally related to the previously adjusted telemetry frequency, and has a predetermined error margin by which the time of a subsequent detection can exceed, or fall short of, the primary duration. If a noise event occurring at one edge of the time window is detected, rather than a signal event occurring more near the center of the window, it is possible for subsequent detection windows, which are re-timed subsequent to the detection of events in the synchronization pattern, to be reset for time intervals which will miss the probable time position of later signal events, leading to loss of synchronization.
It is an object of the present invention to provide a synchronization method for pulse-based telemetry which is more reliable in the presence of noise in the telemetry channel.
It is a further object of the present invention to provide a means for digitally adjusting the telemetry receiver frequency, which can improve the synchronization by eliminating the drift which is present in analog telemetry frequency adjustment circuitry.